First commit of the xroot plugin and client

cmake/Findxrootd.cmake

xroot_clients/ArchiveCmd.cpp

xroot_clients/ArchiveCmd.hpp

xroot_clients/CMakeLists.txt

xroot_clients/xrd_pro_client.cpp

xroot_plugins/CMakeLists.txt

xroot_plugins/XrdProFilesystem.cpp

xroot_plugins/XrdProFilesystem.hpp

xroot_plugins/xrd.cf.fst

xroot_utils/CMakeLists.txt

xroot_utils/ParsedArchiveCmdLine.cpp

xroot_utils/ParsedArchiveCmdLine.hpp